Patterns of family formation have changed dramatically over the decades since the Second World War: Rates of divorce and separation have risen almost linearly; first marriages have shifted to older ages; and more couples begin their relationships with cohabitation rather than marriage. This project is intended to improve understanding of the influence that cohabitation has on the occurrence and timing of marriage, fertility, and marital disruption and that children have on stability of marriages. We will address two basic questions: (1) Are consensual unions just like marriage without the legality or are they fundamentally different? (2) What are the reasons that people choose cohabitation rather than marriage, and why do some dissolve these unions and some proceed to marry? We test two key hypotheses that together bear on these questions: (a) Cohabitations follow the same process of disruption that marriages do. (b) Fertility within consensual unions follows the same process as childbearing within marriage. We see the initial choice as depending, in part, on choices about prospective parenthood and on the person's view of the future of the relationship. If we find no difference between consensual and marital unions in patterns of stability and patterns of childbearing, then we have substantial evidence that, in two very important ways, cohabitational unions are equivalent to marriage. We address these issues of initial choice, stability, and fertility in a series of models that, together will provide detailed picture of the role of consensual unions in contemporary U.S. society. Data for the study come from the National Longitudinal Study of the Class of 1972 and the Panel Study of Income Dynamics,